There are many systems for controlling the defrost operation of the outdoor coil of a refrigeration heat pump apparatus. Experience has traditionally found on heat pumps that a time defrost initiated cycle once every 60 or 90 minutes of elapsed compressor run time is optimum for the worst case when the outdoor temperature is below freezing. The amount of frost during this worst condition is such that the blockage of the outdoor coil is approximately 75%. During times when the outdoor conditions are such that the outdoor coil does not become this blocked, that is, low outdoor humidity, or during cold weather, such frequency of defrost cycling is more often than required. While the air pressure drop through an outdoor coil when the coil is blocked with frost has been used for a defrost control system such as shown in U.S. Pat. No. 3,077,747 issued Feb. 19, 1963, to C. E. Johnson, Jr.; U.S. Pat. No. 3,107,499 issued Oct. 22, 1963, to V. J. Jokela; U.S. Pat. No. 3,062,019 issued Nov. 6, 1962, to L. J. Jungemann, et al; and U.S. Pat. No. 3,066,496 issued Dec. 4, 1962, to V. J. Jokela, often a large pressure drop exists through the outdoor coil when the coil is free of frost. This might be caused by foreign contamination such as dirt, leaves or paper, such things as coil design, that is thin spacing, thin geometry and surface area of the coil and the fan characteristics which affects this pressure drop. The pressure drop also may be quite small as in the case of a high Energy Efficiency Ratio (EER) heat pump where the outdoor coil might be relatively large. Further, the pressure drop can be varied from unit to unit by the outdoor cabinet design which includes leakage of air that may bypass the coil.
All of these systems have a common deficiency in that the systems need to be tailored to a particular heat pump design and to the particular weather conditions. The present invention is concerned with a system to overcome the need of special factory calibration or field adjustment on a demand defrost control.
Specifically, the present invention is concerned with a defrost control system for a refrigeration heat pump wherein the differential pressure is measured across the outdoor coil during a plurality of time controlled operations such as 90 minutes of elapsed compressor operation time, and the highest differential pressure attained during a time controlled operation is used to control the length of normal total compressor operations in a pressure controlled operation before a defrost cycle is accomplished. The heat pump is operated for an extended time period which is selected to be long enough that frosting would occur under any adverse conditions and the differential pressure at the end of that timed operation is measured and stored in a memory. For subsequent operations in between the periodic time controlled operations, the normal operation of the heat pump is accomplished from the space thermostat in pressure controlled operation until the differential pressure across the outdoor coil due to frost reaches a value of that stored in the memory. At that time a defrost cycle is commenced. The differential pressure used for terminating the normal cyclic operation to start the defrost cycle is updated by periodic time controlled operations.